The goal of this project is to elucidate cellular mechanisms by which phosphate (Pi) reabsorption in proximal tubules is regulated at luminal brush border membrane (BBM). The investigations are based on the hypothesis which proposes that regulation of Na+-dependent BBM transport of Pi involves nicotinamide adenine dinucleotide (NAD) and gluconeogenesis (GNG), as major regulatory components. Specifically, we will determine: 1. How the rate of GNG, possibly via the NAD system, modulates GGM uptake of Pi. 2. The mechanism by which NAD interacts with BBM and how this interaction relates to the inhibitory effect of NAD on BBM transport of Pi. 3. How parathyroid hormone (PTH), via cyclic AMP, protein kinase, and Ca++ fluxes stimulates GNG, increses NAD+ generation and thereby inhibits the BBM transport of Pi. 4. Whether gluccorcorticoid administration decreases and dietary Pi deprivation increases BBM uptake of Pi via the effect of GNG and on NAD+ levels. 5. Determine the biochemical mechanism by which nicotinamide administration causes inhibition of BBM uptake of Pi and phosphaturia. 6. To localize the above specified components and regulatory sequences in specific subsegments of proximal tubules, in intracellular compartments of tubular cells, and to determine the differences between superficial and juxtamedullary cortical proximal tubules. These studies, conducted with a combination of biochemical, cellular and microdissection techniques, will elucidate the fundamental principles of regulatio of Pi reabsorption in proximal tubules. The results will also provide a new basis for understanding the pathogenesis of disorders of renal tubular Pi transport and thus will help to establish rationale for new diagnostic and therapeutic procedures in these disorders.